The present invention relates to a method for forming epitaxial wafers from a silicon substrate for use in the fabrication of semiconductor devices and, more particularly, to a method for forming epitaxial wafers which results in significantly increased device yields, particularly for surface fabricated devices such as Schottkey barrier diodes and rectifiers.
Semiconductor devices, including surface fabricated devices such as Schottky barrier diodes and rectifiers, are formed on wafers of epitaxial material which are thereafter diced into chips. The yield, that is, the number of satisfactorily operating chips, as compared to the total number of chips fabricated, is of great importance to a chip manufacturer as the yield, in part, determines the manufacturer's cost per chip.
The yield is determined by various considerations including the degradation of the silicon material of the wafer upon which the devices are formed. The tendency of the silicon material to degrade, in turn, depends upon a variety of factors including impurity levels within the material surface, that is, the concentration of oxygen, carbon, heavy metal and mobile ions. Surface strain and certain surface defects, such as stacking faults, dislocation and vacancies, also contribute to degradation.
The present invention relates to a method for forming epitaxial wafers which controls and minimizes the various factors which contribute to the degradation of the silicon material, thereby reducing failures in the devices formed on the material and, consequently, increasing yields, as well as increasing the performance and overall quality of the finished devices. The method of the present invention includes a number of steps which individually control the various factors contributing to degradation and cumulatively result in a synergistic effect which minimizes the degradation factors to an unexpected degree, thereby increasing the yields to an unanticipated extent.
In general, the process begins with a silicon substrate having a specified oxygen and carbon content, the oxygen acting as an intrinsic getter during device processing. The substrate is sliced into wafers. The backside surface of each wafer is mechanically damaged prior to polishing. The damage sites act as an external getter to remove heavy metals from the electrical junction site in subsequent temperature cycling. The polished substrate is annealed to create a denuded zone so as to reduce strain and defects near the surface which tend to degrade the electrical characteristics of the devices.
A unique two-step epitaxial growth process is utilized to minimize strain, remove mobile ions, and further enhance the denuded zone. An oxide layer is formed by a unique process, preferably immediately prior to device processing. Oxidation is performed after removal of a portion of the epitaxial layer so as to further minimize degradation of the silicon/silicon dioxide interface.
The two-step epitaxial growth process and the process used to form the oxide layer are believed to be each novel in and of themselves, and each of these steps individually, when used in conventional processes, leads to higher yields. However, when these steps are used in combination, an unexpected result is obtained leading to an unanticipated increase in device yield. It is believed that this unexpected result is obtained from the synergistic effect of the combination of these individual steps.
It is, therefore, a prime object of the present invention to provide a method of forming epitaxial wafers which results in significantly increased yields of electrical devices.
It is another object of the present invention to provide a method of forming an epitaxial wafer which employs a unique two-step epitaxial layer formation process.
It is another object of the present invention to provide a method for forming epitaxial wafers which includes a unique oxide layer formation step in which a portion of the epitaxial layer is removed prior to oxidation.
It is another object of the present invention to provide a method for forming an epitaxial wafer wherein the surface of the silicon wafer is subjected to a temperature cycling step for purposes of annealing prior to the formation of the epitaxial layer.
It is another object of the present invention to provide a method of forming epitaxial wafers in which the backside surface of the wafer is mechanically damaged for external gettering.
It is another object of the present invention to provide a method for forming an epitaxial wafer on a silicon substrate which has a specified oxygen content to provide intrinsic gettering during device processing.